A sub frame attached to a vehicle-body rear portion from below comprises a pair of right-and-left side members and a front cross member which interconnects substantially respective central portions, in a vehicle longitudinal direction, of the side members. These transformers are used to increase the voltage of a digitally encoded signal in a stepwise fashion prior to being coupled into a power line. The read head is positioned at a further subsequent position relative to the track of data and further subsequent track signals are obtained The further subsequent track signals are filtered.